Electrostatic printing



United States Patent 3,076,722 ELECTROSTATIC PRINTING Harold G. Greig, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware N0 Drawing. Filed Apr. 29, 1959, Ser. No. 809,612 7 Claims. (Cl. 11737) This invention relates generally to electrostatic printing. More particularly, it relates to improved materials and methods for developing electrostatic images.

'In the art of electrostatic printing, electrostatic images are produced on the surface of an insulating material. Such images comprise a pattern of electrostatic charges on the surface. Visible images are commonly produced therefrom by cascading across the surface a dry mixture of finely-divided developer particles and substantially larger carrier particles. When the developer particles are triboelectrically charged in the opposite polarity to the electrostatic charges, they deposit in charged areas to produce a visible image in substantial configuration with the pattern of charges. When the developer particles have the same polarity as the electrostatic charges, a visible image is produced in reverse configuration with respect to the pattern of charges.

The foregoing method of developing electrostatic images is described in Electrofax Direct Electrophotographic Printing on Paper, by C. J. Young and H. G'. Greig, RCA Review, December 1954, vol. XV, No. 4. Also described in that publication are other methods of development such as: powder cloud, liquid mist and magnetic brush types.

The recording element may comprise almost any insulating surface but, preferably, the recording surface is also photoconductive to enable the recording of light images. Recording elements comprising photoconductive selenium coated plates are described in US. Patent 2,297,691, issued October 6, 1942, to C. F. Carlson. Recording elements comprising photoconductive coatings on paper are described in the Young and Greig publication,

Recently, a so called liquid process for developing electrostatic images has been proposed in which the solid developer particles are suspended in an insulating carrier liquid. Liquid development methods provide many distinct advantages over the use of dry developer mixtures and other methods of developing electrostatic images, for some applications. Basically, the liquid developer previously described consists of finely-divided developer particles dispersed in a hydrocarbon liquid. This developer can be flowed over a surface bearing an electrostatic image or the surface can be immersed in a tray of liquid developer. It can also be sprayed or rolled on to the surface. When appropriate developer particles are dispersed in a properly selected liquid, they acquire an electrophoretic or triboelectric charge enabling them to be attracted to an electrostatic charge pattern of appropriate polarity. Deposition of the developer particles on the charge image is an example of the phenomenon known as electrophoresis or cataphoresis. A liquid developer process for charge images is described in greater detail by K. A. Metcalf and R. J. Wright in a paper entitled Xerography, published in the Journal of the Oil and Colour Chemists Association, November 1956, volume thereof are generally toxic.

3,075,722 Patented Feb. 5, 1963 39, No. 11, London, England and in another paper entitled Liquid Developers for Xerography, published in the Journal of Scientific Instruments, February 1955, volume 32.

Although the above-mentioned liquid developer compositions are suitable for many purposes, they do possess undesirable properties. Most hydrocarbon liquids are solvents for developer powders which include resins, Waxes, or organic pigments. When resinous particles are dispersed in a hydrocarbon liquid, they dissolve to some extent so that they become tacky and tend to agglomerate.

plished with a fixative spray or by heating. When hydrocarbon carrier liquids are employed, heating can be extremely dangerous in view of the fire hazard involved. In addition to this, whether heated or not, most hydrocarbon liquids have an objectionable odor and the vapors For the foregoing reasons it can be readily seen that such liquids are unsuitable for many applications such as, for example, office copiers.

Accordingly, it is a general object of this invention to provide improved compositions of matter for developing electrostatic images.

It is a further object of this invention to provide improved relatively non-toxic and non-inflammable compositions of matter for developing electrostatic images.

It is a further object of this invention to provide an improved composition of matter for developing electrostatic images on a surface, which composition includes developer particles that automatically become fixed to the surface immediately after development of the image.

It is yet another object of this invention to provide an improved composition of matter for developing electrostatic images, which composition will produce a visible image when applied to a negative electrostatic image, said visible image being in reverse configuration with respect to the negative electrostatic image.

It is still a further object of this invention to provide improved methods for developing electrostatic images, which methods obviate any need for a fixing step.

The foregoing objects and other advantages are accomplished in accordance with this invention which provides improved compositions for developing electrostatic images. The compositions comprise finely-divided electroscopic developer particles dispersed as a phase in a liquid carrier which also has dissolved therein a binder material for the developer particles. The liquid carrier comprises an electrically-insulating liquid which has a viscosity of about 0.4 to 1 centipoise at ordinary room temperatures, which has an evaporation rate substantially greater than that of toluene, and which has a flash point in excess of 200 C. Also contemplated is the develop ment of electrostatic images by applying thereto the above liquid dispersion, causing developer particles to deposit from the liquid onto the surface by electrostatic attraction, and evaporating the liquid carrier from the surface leaving the particles fixed thereon by the binder material.

Specific examples and additional advantages of the developer compositions and of the improved methods of developing electrostatic images in accordance with this invention are included in the detailed description which follows.

CARRIER LIQUIDS An important feature of this invention is the provision of an evaporable carrier liquid which is relatively nontoxic, non-inflammable and has a low viscosity. Such a liquid makes feasible the use of a liquid developer composition in applications such as, for example, office copiers without any need for exhaust equipment to remove noxious vapors. In any such application, however, a well ventilated work area should be provided when development equipment is to be used continuously for extended periods of time. In accordance with this invention, such a liquid preferably comprises a fluori'nated, chlorinated hydrocarbon which has about equal numbers of fluorine and chlorine atoms. A specific example of such a liquid is trichlorotrifluoroethane. This liquid is a very selective sol.- vent which will not dissolve most resins and waxes. It has a viscosity of about 0.69 centipoise at room temperature and has an evaporation rate substantially in excess of that of toluene. It is far less toxic than materials such as gasoline or carbon tetrachloride or for that matter almost any organic solvent. It has a flash point in excess of 650 C. Because of its low toxicity and high flash point, it can be employed in ordinary room atmosphere with practically no danger to health and without fire hazard.

DEVELOPER COMPOSITIONS To prepare a suitable developer composition, finely divided particles of electroscopic developer material, insoluble in the liquid carrier, are dispersed therein in proportions such that up to about 3% by weight of the composition consists of developer particles. Also included in the composition is a binder material for the developer particles, this binder material being dissolved in the liquid 7 carrier in a concentration such that up to about 5% by weight of the composition consists of binder material.

A specific example of a suitable developer composition is as follows:

Example I A black pigment is prepared by making two solutions; solution one comprises:

6 grams Iosol Black (C.I. Solvent Black 13) 400 grams methanol solution two comprises:

9 grams Spirit Nigrosine (CI. 50415) 4-00 to 600 grams methanol Solution one is poured into solution two with continuous stirring. Once the solutions have been thoroughly mixed, and a black relatively insoluble pigment is precipitated, the mixture is filtered and the filter cake allowed to dry. The dried filter cake is broken up and dispersed in dimethyl polysiloxane liquid having a viscosity of about 2 centistokes. The proportions in this dispersion are about 1 to 8 parts black pigment to about parts of liquid. It is preferred that the liquid content be kept as low as possible but sufiicient to provide a uniform dispersion. After ball milling, the black pigment is classified as to particle size. Particles having a diameter of 74 microns or less are preferred.

Also prepared is a solution of a polyisobutylmethacrylate binder material in a ratio of:

2 /2 grams polyisobutylmethacrylate 200 grams trichlorotrifluoroethane When the polyisobutylrnethacrylate is dissolved, about one part by weight pigment dispersion per 200 parts solution is added to provide a final developer composition. Although a specific ratio of binder material to carrier material is specified herein the proportions may vary over a considerable range. With a minimum of about .1% by weight polyisobutylmethacrylate in the trichlorotrifluoroethane a developed image can be fixed. The above proportions will permit an image so produced to be overprinted with a second image of any desired different color. However, when a gloss surface is desired, this minimum proportion is insufiicient. To provide such a gloss surface it is desirable to employ a saturated solution of binder material in carrier liquid. In the case of polyisobutylmethacrylate in trichlorotrifluoroethane, the composition comprises about 2 /2 by weightv of binder material.

BINDER MATERIALS AND PIGMENTS In addition to the polyisobutylmethacrylate specified above, many other binder materials may be employed, it only being necessary that the binder material selected be soluble in the selected carrier. liquid. Specific examples of binder materials which may be dissolved in trichlorotrifluoroethane include the following:

( l Diphenyls (2) Chlorinated biphenyls and polyphenyls (3) Ester gums (4) Gum mastic (5 Stearic acid (6) Resinous polysiloxanes (7) Parafiins (8) Piccolytes (those having melting points between 50 C. and C.-a thermoplastic terpene resin-Pennsylvania Industrial Chemical Corp, Clairton, Pa.).

In addition to the black pigments specified in Example 1, many colored or metallic pigments may also be employed. Here also there is a specific criterion in that a pigment must be selected which is insoluble in the selected carrier fluid. Among the pigments which are suitable for use in trichlorotrifluoroethane are the following:

(1) Cyan Blue Toner GT (described in U.S. 2,486,351

to R. H. Wiswall, Jr.) (2) Benzidene Yellow (3) Sudan 3 Red (Color Index No. 2-6100) (4) Hansa Yellow G (Color Index No. 11680) (5) Pyrazolone red and maroon pigments (6) Carbon blacks (7) Powdered metals Any of these pigments may be dispersed in the dimethyl polysiloxane as was the filter cake material exemplified in Example I. When so dispersed, the pigment dispersion is mixed into the carrier liquid in proportions the same as those described for the filter cake dispersion. Alternatively, pigments which are sufficiently finely-divided may be mixed into the carrier liquid directly by any standard means, no ball milling being required. Where the pigment materials are not sufiiciently finely-divided, they may be dispersed in the carrier liquid and ball milled therein to reduce the particle size, provided that ball milling is carried out in a closed system to prevent evaporation of the carrier liquid. Another alternative comprises melting the binder material, dispersing the pigment therein, and reducing the hardened melt to a powder of desired particle size. Mixing the powder in the liquid carrier will cause the binder material to dissolve whilev the pigment particles become dispersed therein.

Any of the foregoing developer compositions may be applied to an electrostatic image consisting of a pattern of negative electrostatic charges. Particles suspended in the carrier liquid are attracted by the negative charges and deposited on the pattern of such charges. In such a process, these compositions therefore constitute direct developer compositions.

When the compositions are employed to develop electrostatic images consisting of patterns of positive electrostatic charges they will be repelled by the charges and will deposit in non-charged areas of the electrostatic image to produce a reverse visible image and hence can be called reverse developer compositions in such processes.

In any electrostatic printing process the carrier liquid in the compositions of the present invention will rapidly evaporate, usually in a few seconds, and will leave hehind on the electrostatic image developer particles which are fixed in place by the binder material.

REVERSE DEVELOPER COMPOSITIONS In accordance with this invention, developer compositions which, when applied to a pattern of negative elect'rostatic charges, produce a reverse visble image are also provided. Such a developer composition may be prepared by dispersing a pigment in a binder material which has a triboelectrically negative character with respect to the carrier fluid and the negative charges of the electrostatic image. Such a binder material is one which is predominantly comprised of polyvinyl chloride. Preparation of such a composition may be carried out as follows:

Example II 1 gram of carbon black grams Vinylite VYNV (a copolymer of about 96% vinyl chloride, 4% vinyl acetate) grams of dimethyl polysiloxane (viscosity about 2 centistokes) This mixture is ball milled for about 16 to hours. A solution of binder material in carrier liquid is then prepared in proportions of:

2 /2 grams polyisobutylmethacrylate 300 grams trichlorotrifluoroethane electrical properties of the resin must not be present. In

almost all applications, the resin will comprise at least 90% by weight vinyl chloride. When such a resin comprises 96% by weight vinyl chloride and 4% vinyl acetate, excellent reversal development is achieved.

Color dispersions may be prepared in a like manner employing most of the pigments discussed heretofore. In addition to the afore-mentioned pigments, those which are soluble in the carrier liquid may also be employed when they are first dispersed in a polyvinyl chloride resin which is insoluble in the carrier liquid. These pigments need not necessarily be ball milled in dimethyl polysiloxane but may be, instead, dispersed in a hot melt of the polyvinyl chloride resin and then reduced to powder form of the desired fineness to produce insoluble developer particles.

The developer compositions of Example II have been described as being reversal type compositions for the development of negative patterns of electrostatic charges. They may also be employed to develop patterns of positive electrostatic charges in which case they deposit on charged areas to produce a direct visible image.

Use of any of the foregoing developer compositions in electrostatic printing processes, as contemplated in this invention, provides for new and substantially improved results. In accordance with this invention, the methods call for applying a developer composition to an electrostatic image by such means as, for example, flowing across the image, spraying, application with a roller, or by immersing the image in a tray containing the liquid composition. When an electrostatic image is developed in this manner, the improved results are immediately evidenced in that a sharply defined image is obtained with minimum deposition of developer particles in non-image areas. Within a few seconds after development, the surface on which developer particles were deposited is dried and the developer particles are fixed thereto by the binder material.

When a low proportion of binder material is employed, as suggested herein, a color image may be overprinted with a second color image and again overprinted with a third (or more) to provide a composite image in as many colors as desired and, at the same time, to provide for color mixing. When saturated solutions of binder material are employed as suggested herein, a visible image is produced and the image surface will have a high gloss.

The images produced by the methods of this invention are durable and can withstand repeated handlings without smearing. When produced on a flexible surface the visible image will flex with that surface rather than peel or chip therefrom.

What is claimed is:

1. A composition of matter consisting essentially of a carrier liquid consisting essentially of trichlorotrifluoroethane, electroscopic organic pigment particles dispersed as a phase in said liquid, and a separate binder material for said pigment consisting essentially of a polyisobutylmethacrylate dissolved in said liquid, said binder material comprising about 0.1 to 5.0% by weight of said composition.

2. The composition of claim 1 wherein the concentration of said pigment particles in said composition is not in excess of about 3 by weight.

3. The composition of claim 1 wherein the concentration of said pigment particles in said composition is about 0.25% by weight and the concentration of said binder material in said composition is about 0.70% by weight.

4. A composition of matter consisting essentially of a carrier liquid consisting essentially of trichlorotrifluoroethane, finely-divided electroscopic developer particles dispersed as a phase in said liquid, said developer particles consisting essentially of about 5 to 10 parts by weight of resin and about one part by weight of color pigment, said resin consisting essentially of a copolymer of about 96% by weight of vinyl chloride and 4% by weight of vinyl acetate and a separate binder material for said developer particles dissolved in said liquid, said binder material, comprising about 0.1 to 5.0% by weight of said composition, and consisting essentially of a polyisobutylmethacrylate.

5. The composition of claim 4 wherein the concentration of said developer particles in said composition is not in excess of about 3% by weight.

6. In a method of developing an electrostatic image on an insulating surface said image comprising a pattern of negative electrostatic charges, the improvement comprising applying to said surface a developer composition consisting essentially of pigment electroscopic developer particles dispersed as a phase in trichlorotrifluoroethane and a polyisobutylmethacrylate binder material dissolved in said trichlorotrifluoroethane, said binder material comprising about -0.1 to 5.0% by weight of said composition, depositing said developer particles on said pattern of negative electrostatic charges, and binding said developer particles to said surface with said polyisobutylmethacrylate by evaporating said trichlorotrifluoroethane from said surface.

7. In a method of developing an electrostatic image on an insulating surface, said image including areas bearing negative electrostatic charges, the improvement comprising: applying to said surface a developer composition consisting essentially of pigmented electroscopic developer particles the major proportion of which is vinyl chloride dispersed as a phase in trichlorotrifluoroethane and a polyisobutylmethacrylate binder material dissolved in said trichlorotrifluoroethane, said binder material comprising about 0.1 to 5.0% by weight of said composition, de 2,735,784 .G-reig et al. Feb. 21, 1956 positing said. developer particles in areas other than said 2,773,855 Hochberg et a1 Dec. 11, 1956 areas bearing negative electrostatic charges, and binding 2,892,794 Insalaco June 30, 1959 said developer particles to said surface with said po1y- 2,898,279 Metcalfe et a1. i Aug. 4, 1959 isobutylmethacrylate by evaporating said trichlorotri- 5 2,899,335 Straughan Aug. 11, 1959 fiuoroethane from said surface. 2,907,674 Metcalfe et a1. Oct. 6, 1959 2,940,934 Carlson June 14, 1960 References Cited in the file of this patent 3,010,842 Ricker V. Nov. 28, 1961 UNITED STATES PATENTS 3,038,799 Metcalfe et a1, June 12, 1962 2,297,691 Carlson Oct. 6, 1942 10 OTHER REFERENCES 2,381,753 Irion i Aug. 7, 19.45 Zimmerman, etv 211.: Handbook of Material Trade 2,731,436 Stetz et a1 Jan. 17, 1956 Names, 1953, p. 

6. IN A METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE ON AN INSULATNG SURFACE SAID IMAGE COMPRISING A PATTERN OF NEGATIVE ELECTROSTATIC CHARGES, THE IMPROVEMENT COMPRISING APPLYING TO SAID SURFACE A DEVELOPER COMPOSITIONS CONSISTING ESSENTIALLY OF PIGMENT ELECTROSCOPIC DEVELOPER PARTICLES DISPERSED AS A PHASE IN TRICHLOROTRIFLUOROETHANE AND A POLYISOBUTYLMETHACTYLTE BINDER MATERIAL DISSOLVED IN SAID TRICHLOROTRIFULOROETHANE, SAID BINDER MATERIAL COMPRISING ABOUT 0.1 TO 5.0% BY WEIGHT OF SAID COMPOSITION, DEPOSITING SAID DEVELOPER PARTICLES ON SAID PATTERN OF NEGATIVE ELECTROSTATIC CHARGES, AND BINDING SAID DEVELOPER LATE BY EVAPORATING SAID TRICHLOROTRIFLUOROETHANE FROM SAID SURFACE. 